Black garden ants reduce aphid predation by native and invasive ladybirds

Abstract Intraguild interference between predators can hamper top‐down regulation. For example, aphid‐tending ants often defend their mutualistic partners against antagonists. Greater ant aggression against native ladybirds compared to non‐native ladybirds might benefit the non‐native predator, due to predator–predator naïveté. As a consequence, non‐native ladybirds are expected to consume more aphids and might thus outcompete native ladybirds. We established microcosms with a bean plant ( Vicia faba ) infested with black bean aphids ( Aphis fabae ). The microcosms were divided into the following treatments: no ants, aphids tended by common red ants ( Myrmica rubra ) and aphids tended by black garden ants ( Lasius niger ). We added either a native ladybird ( Coccinella septempunctata ) or an invasive ladybird ( Harmonia axyridis ) to the microcosms. We compared deterrence of ladybirds from plant‐based aphids by ants and the number of aphids remaining at the end of the experiment. Our results indicate that L. niger , but not M. rubra , deterred ladybirds from the plant and strongly increased the number of remaining aphids in the presence of both ladybird species. In contrast to our expectation, more aphids tended to remain in the presence of H. axyridis and M. rubra compared to the C. septempunctata and M. rubra combination. We conclude that intraguild interference can weaken top‐down control and depends on the aggressiveness and potentially prey consumption of competing species. However, we found no support for the predator–predator naïveté hypothesis suggesting that non‐native predators would be released from intraguild interference and thus consume more prey.